"Remove branding" from record also breaks it

[alejandromumo]

Package version (if known): v17.4.0

Describe the bug

When branding is removed from a record, the record rendering returns an HTTP 500 Internal Server Error and it does not render anymore

Steps to Reproduce

1. Publish a record and add it to a community
2. Click on "Manage communities" on the record landing page
3. Click "Remove branding"
4. Refresh the record landing page
5. The record is broken. Loading the record displays an Internal Server Error

Expected behavior

The record should be rendered without any branding.

Additional context

This was initially reported by @mug0m3s in zenodo-rdm

Investigation so far

After the branding is removed, the record.parent.communities.default is set to '' which then causes the error in the backend when reading the record again:

(builtins.ValueError) badly formed hexadecimal UUID string.

I believe the fix is either to remove the default entry at all or set it to None, to be tested

[slint]

Closed via the fix at inveniosoftware/invenio-app-rdm#2869